The invention relates to a resistance measurement method and to a circuit arrangement suitable for performing the method.
Such methods and circuits can e.g. be used for determining resistance values of strain gauges and therefore for the measurement of strains, e.g. for determining forces, pressures or torques, or also for producing temperature-dependent signals.
Particularly in connection with temperature measurements, it is known to feed a temperature-dependent resistor with a precisely known constant current and to digitize the resulting voltage drop at the resistor by means of a precise analog-digital converter. This requires high quality and therefore very expensive analog components, whose characteristic curve must not shift significantly over a wide temperature range.
It is also known in a first step to set a current of a constant current source with the aid of a reference resistor, to integrate a voltage up to a certain, specific value on a capacitor by means thereof and to measure with a counter the time by which said voltage value is reached and then to store the count. In a second step the constant current is set and inverted with a temperature-dependent measuring resistor and with said current the capacitor is discharged. Once again the time up to the complete discharge of the capacitor is measured and the corresponding count established. The ratio of the two counts is the ratio of the two resistors, which gives the measured temperature. Once again very precise and therefore expensive equipment components are needed, because otherwise the influences of errors would very rapidly become intolerable.
DE 36 42 862 C2 discloses a circuit arrangement for producing a temperature-dependent signal, a reference resistor and a temperature-dependent resistor being provided, by means of which a reference time and a measurement time can be produced and from the comparison of these times can be derived a digital signal associated with the measured temperature. For this purpose a charging capacitor is alternately charged across the reference resistor and the temperature-dependent resistor to the same, predetermined threshold values, the charging times are measured, so that in the case of a known reference resistance from the ratio it is possible to determine the value of the temperature-dependent measurement resistance and therefore a temperature value.
The advantage is this circuit and the corresponding method compared with the known types is that no analog components are required and no special demands are made on the quality of the digital components. However, it is disadvantageous that the measurement takes place during the charging process when a relatively high current must be flowed and in many possible uses this has to be supplied by a battery or a solar module. The internal resistance of the battery or module gives rise to an interfering influence with respect to the resistance determination of the reference and measuring resistor.
Another essential disadvantage is that by means of the determination of the charging time it is not the resistance value of the reference or measuring resistor alone, but instead the resistance value as such is measured, which is given by the indicated resistance and the internal resistance of the electronic switch/transistor connected in series therewith and which is unknown. For as long as the resistance value of the switch, which with CMOS transistors is 10 to 20 ohms with respect to the resistance value of the reference and measuring resistor and with NTC (Negative Temperature Coefficient) resistors is in the range 15 to 20 kohms, is negligible the known method and circuit operate in a satisfactory manner. However, for highly precise temperature measurements it is not possible to use such NTC resistors due to their non-linearity and lack of long-term stability. It is then necessary to use platinum resistors, which have much lower resistance values of approximately 100 to 500 ohms. Thus, as a result of th is with the aforementioned switch resistance values a significant error arises, particularly if it is borne in mind that a platinum resistor in the case of a temperature change of 1xc2x0 C., changes its resistance value by only 0.4%. The indicated problem can also not be solved by the use of expensive, external power MOSFETs. The latter admittedly have the resistance value of approximately 10 to 15 mohms (milli-ohms), but with a platinum resistor PT100 a temperature change of 0.01xc2x0 C. leads to a resistance change of 4 mohms and which is therefore of the order of magnitude of the resistance of said MOSFET switch. In addition, such power MOSFETs are not only expensive, but are also unsuitable for measurement electronics for other reasons.
An important disadvantage of strain gauges is the limited shift in the resistance change of such strain gauges. Typically the resistance of a strain gauge varies by approximately 0.2% from 0 to full scale deflection. This value is normally expressed as a change in parts per million (ppm). 0.2% corresponds to the value of 2,000 ppm. Thus, a strain gauge typically has a 2,000 ppm shift. As opposed to this a typical temperature-dependent, platinum resistor (e.g. PT500) changes its resistance per temperature difference degree by 3920 ppm or for a typical temperature shift of 100% by 392000 ppm or 39.2%. Thus, it is not readily possible to transfer to strain gauges methods known from temperature measurements, because the strain gauge shift is approximately 200 times lower. In addition, with such small measurement quantities, a decisive part is increasingly played by the unavoidable noise effects of the electronic components or circuits used, which leads to an additional deterioration of the measurement precision and resolution of a method or circuit arrangement for measuring such quantities.
With time or time-resolved measuring methods, which are used for avoiding the influences of different curve shapes of the measurement signals of a threshold switch, e.g. a Schmitt trigger, due to the time lag of the threshold switch a further problem arises, because such a lag cannot generally be ignored. It is particularly noticeable with strain gauges in the measured result with values of up to 10 ppm. As it is also highly dependent on the temperature and voltage, the threshold switch time lag is also noticeable as a temperature error.
Known electrical resistance measurement methods, such as are e.g. known from DE 44 20 998 C2, use signal processing means in the form of processors or rapid counters for determining time intervals.
In the processor sector at present using conventional processes maximum clock frequencies of approximately 20 MHz can be implemented. When using hardware-based, rapid counters this can be raised to approximately 200 MHz. Beyond this value significantly increased costs and high current consumption or power loss make such a device uncompetitive and can only therefore be used to a limited extent as a result of its restricted time resolution.
The problem of the invention is to provide a method and a circuit for the precise measurement of resistances, whilst avoiding the aforementioned disadvantages.
In the case of a method of the aforementioned type, the invention solves this problem in that a capacitor is repeatedly charged and discharged and the charging or discharging time is measured by means of at least one resistor and at least one first switch connected in series therewith, at least one second switch connected in series with the resistor and juxtaposed, parallel-connected switches in series with the resistor, whilst using a threshold switch. The invention also solves the set problem in the case of a circuit arrangement for resistance measurement by providing signal processing means, at least one capacitor and with respect thereto mutually parallel-connected, at least two resistors, a first switch being in each case connected in series with the resistors and wherein at least one second switch is connected in parallel with the first switch.
Thus, according to the invention, the same useful resistance is measured in different combination with the replaceable, parasitic switch resistances, so that the interfering part of the latter can be eliminated. According to the invention with the particular resistor, reference and measuring resistors, is not only connected in a single switch, but also a further switch (the switches being connected in parallel to one another). Thus, this permits in the inventive method the performance of discharge measurements not only during the discharge of a resistor across a switch, but also in the case of the discharge across the other switch and during the discharge with simultaneously switched through, two switches and as a result of the measured, different charging times the internal resistances of the switches and therefore the influence thereof on the measured result can be completely eliminated, so that the internal resistances of the switches can play no part when determining the measured result and cannot falsify the latter.
Preferably the time measurement takes place during discharging and not during charging. This makes it possible to perform the charging process with a low current, so that weaker current sources can be used.
An important advantage of the invention is that no expensive and complicated, external transistors have to be used, but instead use can be made of transistors available in processors and other integrated circuits, such as FPGAs or ASICs, so that the overall circuit can be entirely constructed as an integrated circuit and therefore, as stated, requires no external transistors.
With platinum resistors having a 500 ohm resistance value (PT500) it is sufficient to use standard transistors, i.e. so-called 8 mA types, whilst with 100 ohm platinum resistors at least 24 mA standard transistors should be used, but once again they can be implemented by the parallel connection of several 8 mA transistors.
Unlike in the case of the prior art, interfering effects are not merely minimized, but instead completely eliminated as a result of the circuit and measuring method according to the invention. The determination of the resistance values of reference and measuring resistors to be used, whilst eliminating internal resistances of the switches can be performed rapidly in current-saving manner using processors conventionally available or special arithmetic circuits in integrated circuits (ASICs or FPGAs). The resistance measurement according to the invention can be implemented inexpensively and requires no analog components. It is extremely voltage-stable and temperature-stable. The counters can be constituted by time-to-digital converters or TDCs, whose current consumption is lowerable by more than a power of 10 compared with that of existing solutions.
Whereas in the aforementioned inventive method for determining the resistance, the capacitance of the capacitor must be known, according to a preferred development of the invention the aforementioned measurements and determinations are repeatedly performed over two or more resistances and for determining the resistance ratio of two resistances or resistors, the results are in each case divided by one another.
In order when using the method of the invention with strain gauges to obtain a high accuracy of measurement, according to a further development thereof, the determination of the resistance Ri (i=1, 2, 3) takes place by means of a relation, particularly                               C          ·          Ri                =                  xe2x80x83                ⁢                  ti          -                                                    ti                2                            +                              ti1                ·                ti2                            -                              ti1                ·                ti                            -                              ti2                ·                ti                                                                            =                  xe2x80x83                ⁢                  M          +          K                    
containing a measurement term (M) and a correction term (K). The correction term represents the time fraction of the internal resistances of the switching transistors. This correction term does not change with the value to be measured, specifically the strain of the strain gauges, but instead only with the change in the internal resistances, i.e. with temperature or voltage fluctuations. As these are only slow changes, it is possible to very highly average the correction term and therefore largely free it from noise effects. Thus, according to a preferred development of the method of the invention, the correction term and the measurement term are averaged with different averaging types and also the correction term is averaged higher by a specific factor, particularly between 32 and 64, than the measurement term.
Thus, the noise of the correction term is reduced almost to zero relative to the measurement term. The lower averaging of the measurement term ensures that also short periodic changes of the measured value, e.g. in the case of oscillation or vibration measurements, are not lost.
In order to eliminate from the measurement results the problematic time lag of the threshold switch (Schmitt trigger), according to the inventive method the measured times are reduced by a time lag of the threshold switch. For this purpose the time lag is determined by measuring the time of in each case a capacitor charge or discharge across a first resistor and a second resistor, as well as a parallel connection of both resistors by closing a first switch, a second switch or both switches.
The time lag of the Schmitt trigger can be subdivided into two. One time interval is dependent on the fundamental parameters of the Schmitt trigger, as well as temperature and voltage influences. The second, smaller-amount time fraction is dependent on the steepness of the input slope of the signal at the Schmitt trigger. In the present case it is an e-function resulting from the charge/discharge characteristic of the capacitor. The first and dominant of the two time fractions can be compensated by the inventive method, so that the measuring errors caused by the threshold switch lag can be significantly reduced.
According to a highly preferred further development of the inventive method, there is a separate determination of the charging/discharging times for resistance measurement and the time lag. Preferably a time-to-digital converter (TDC) is used for time measurement purposes. When using a timing unit in the form of a TDC, compared with the conventionally used processors or hardware counters, it is possible to achieve much higher clock frequencies of  greater than 50 GHz. The method according to the invention operates roughly 250 times faster than the rapid counter variants and is also characterized by a much lower current consumption. Within the framework of the uses where when using a TDC a measured value resolution of 11 bits, i.e. a more than 2,000 times subdivision of the measuring range at a measuring frequency of 100 Hz can be implemented, counter variants only give a measured value every 2.5 seconds and are consequently not competitive. To avoid such disadvantages in known methods, it was necessary to use capacitors with a 250xc3x97 capacitance. Such capacitors were in the range of a few xcexcF, but were unable to fulfil the high linearity demands made for such applications. Such demands are only satisfied by capacitors with special dielectrics (e.g. plastics) unable to effectively implement such high capacitances.
According to a further development of the inventive circuit, at least one of the resistances is a known reference resistance and at least one is an unknown measuring resistance.
A preferred field of use of the invention is the measurement of mechanical quantities using strain gauges, which as a result of strain change their resistance. With strain gauges the particular application significantly decides on how many resistances are measured. Frequently a complete bridge of four resistors or resistances is measured, because only through the mechanical arrangement of the strain gauge resistors on the object to be measured (e.g. a weighing scale pan) is the necessary precision and linearity obtained. In the case of e.g. a complete bridge, all the resistors are measuring resistors and only the mutual ratio thereof gives the measured value.
Another field of use is the heat consumption determination of a heat consuming means. With the temperature measurement necessary here, typically one of the resistors, e.g. R1, is a known reference resistor, whilst by means of the further measuring resistors measurement takes place of the temperature in the forward and also temperature-dependent return movement of a heating means and from the difference of these two temperatures, represented by the resistance values of the measuring resistors, the heat quantity consumed is calculated.